Apparatus and process for hydrogenations

ABSTRACT

An apparatus and process for treating a complex mixture of hydrocarbons containing undesirable olefinic compounds to remove the mono olefins and diolefins in two stages and separate a desirable key component from the mixture, by first treating the key component in a reactive distillation column under mild conditions to hydrogenate diolefins then separating the diolefin-depleted key component and any lighter materials from the heavier components and sending the diolefin-depleted key component and lighter materials to a second reactive distillation column where the lights are removed overhead and the diolefin-depleted key component is hydrogenated under more severe conditions to remove the mono olefins.

This is a division of application Ser. No. 09/262,251 filed on Mar. 4,1997, now U.S. Pat. No. 6,284,104.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and process for improvingthe flexibility of operation of reactive distillation hydrogenationprocesses.

2. Related Art

The use of catalysts in a distillation column to concurrently carry outchemical reactions and separate the reaction products has been practicedfor some time. Surprisingly, this use of a catalytic distillation columnreactor lends itself particularly well for hydrogenations. See forexample, U.S. Pat. Nos. 5,595,634; 5,599,997; 5,628,880; 5,773,670 andEuropean Patent No. 0556025 B1. The combination is useful because thereactants in the liquid phase are quickly separated from the reactionproducts due to boiling point differences by fractional distillation.Thus further reaction is suppressed.

Several different arrangements have been disclosed to achieve thedesired result. For example, British Patents 2,096,603 and 2,096,604disclose placing the catalyst on conventional trays within adistillation column. A series of U.S. patents, including those listedabove and more, particularly U.S. Pat. Nos. 4,443,559 and 4,215,011disclose using the catalyst as part of the packing in a packeddistillation column. The use of multiple beds in a reaction distillationtower is also known and illustrated, for example, in U.S. Pat. Nos.4,950,834; 5,321,163; and 5,595,634.

In reactive distillations, such as catalytic distillation, as in anyother distillation, there is no rigid cut off between the components.Reactions carried on in specified portions of the column using someconstituents may leave undone other desirable treatment of otherportions of the column constituents.

For example, mixed refinery streams often contain a broad spectrum ofolefinic compounds. This is especially true of products from eithercatalytic cracking or thermal cracking processes (pyrolysis gas). Theseunsaturated compounds comprise ethylene, acetylene, propylene,propadiene, methyl acetylene, butenes, butadiene, amylenes, hexenes,etc. Many of these compounds are valuable especially as feed stocks forchemical products. Olefins having more than one double bond and theacetylenic compounds (having a triple bond) have lesser uses and aredetrimental to many of the chemical processes in which the single doublebond compounds are used, for example, polymerization. Sulfur andnitrogen compounds, among others, are frequently desirably removed alsoand they may be effectively removed from a portion of the columnconstituents, but because of different boiling points for other portionsof the column constituents and the contaminants therein, not all of thecontaminants may be removed.

Generally it is more difficult to remove both dienes and olefins thandienes alone. Diene-rich streams will hydrogenate at a higher volumetricrate under milder conditions than will a diene depleted olefinic stream.Sulfur in the several hundred ppm range is not uncommon for some feeds.Palladium hydrogenation catalysts are not able to handle such highsulfur levels, however, double-digit diene levels often present in thesefeeds overwhelm the sulfur impurities in their mutual competition forcatalyst sites thereby providing reasonable rates notwithstanding.

In hydrotreating streams with high concentrations of dienes present(above 1000 ppm), there is a need to refrain from using hightemperatures to avoid oligomerization. Generally, temperatures in thearea of 170° F. or above are avoided. Such operating restraints createconditions which are unfavorable for exhaustive olefin conversion in thesame unit in which the diene is eliminated.

The present invention provides apparatus and process to address thereactive distillation hydrogenation of feed streams havingconcentrations of both mono and di-olefins.

SUMMARY OF THE INVENTION

The present invention includes an apparatus for conducting reactivedistillations comprising a first distillation column, a first primarycatalyst bed for carrying out a hydrogenation of unsaturated compoundscomprising diolefins, said first primary catalyst bed being positionedin said distillation column to provide a first reaction zone fordiolefins in said first distillation column, and optionally, a firstsecondary catalyst bed above said first primary catalyst bed, said firstsecondary catalyst bed to provide a second reaction zone for diolefinsremaining in said first distillation column after said first reactionzone, a first mixed saturated/unsaturated compound feed entry below saidfirst primary bed, a hydrogen feed below said primary bed, a bottomsline and an overhead line connecting to a second distillation columncomprising a second primary catalyst bed for carrying out hydrogenationof unsaturated compounds comprising mono olefins from said firstdistillation column, said second primary catalyst bed being positionedin said distillation column to provide a first reaction zone forunsaturated compounds in said second distillation column, andoptionally, a second secondary catalyst bed below said second primarybed, said second secondary catalyst bed to provide a second reactionzone for mono olefins remaining in the second distillation column aftersaid first reaction zone, said overhead line from said firstdistillation column connecting to said second distillation column abovesaid second primary catalyst bed and a hydrogen feed below said secondprimary bed.

The process carried out in the apparatus is also part of the presentinvention.

There may be distillation structures or trays between the primary andsecondary beds. Hydrogenation reactions liberate a significant heat ofreaction (on the order of 50,000 or greater BTU/lb mole H₂ consumed).This released heat adds to vapor load in the column. Optionally, sidecondensers may be used to keep the uniformity of the vapor profile inthe column within desired ranges. A secondary catalyst bed may bepositioned in the distillation column, above or below the primary bed asheretofore described, to allow lighter or heavier boiling components tobe exposed to additional catalyst and be purified or treated further.

The term “reactive distillation” is used to describe the concurrentreaction and fractionation in a column. For the purposes of the presentinvention, the term “catalytic distillation” includes reactivedistillation and any other process of concurrent reaction and fractionaldistillation in a column regardless of the designation applied thereto.

The catalyst beds as used in the present invention may be described asfixed, meaning positioned in a fixed area of the column and includeexpanded beds and ebulating beds of catalysts. The catalysts in the bedsmay all be the same or different so long as they carry out the functionof hydrogenation as described. Catalysts prepared as distillationstructures are particularly useful in the present invention.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a simplified process flow diagram of one embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is particularly useful for removal of mono olefinsand diolefins from cracked gas streams. Thermally cracked gas streamshave a particularly wide range of carbon numbers and compound types.Normally, compounds in one carbon number range (such as C₄'s in a C₄/C₅splitter) will function as the light key grouping in the column. Thuscompounds one carbon number greater (such as C₅'s in a C₄/C₅ splitter)will serve as the heavy key grouping for the column.

Hydrogenation is the reaction of hydrogen with a carbon-carbon multiplebond to “saturate” or partially saturate the compound. This reaction haslong been known and is usually done at super atmospheric pressures andmoderate temperatures using a large excess of hydrogen over a metalcatalyst. Among the metals known to catalyze the hydrogenation reactionare platinum, rhenium, cobalt, molybdenum, nickel, tungsten andpalladium. Generally, commercial forms of catalysts use supported oxidesof these metals. The oxide is reduced to the active form either prior touse with a reducing agent or during use by the hydrogen in the feed.These metals also catalyze other reactions, most notably dehydrogenationat elevated temperatures. Additionally, they can promote the reaction ofolefinic compounds with themselves or other olefins to produce dimers oroligomers as residence time is increased.

Selective hydrogenation of hydrocarbon compounds has been known forquite some time. Peterson et al in “The Selective Hydrogenation ofPyrolysis Gasoline,” presented to the Petroleum Division of the AmericanChemical Society in September of 1962, discusses the selectivehydrogenation of C₄ and higher diolefins. Boitiaux et al in “NewestHydrogenation Catalyst”, Hydrocarbon Processing, March 1985, presents ageneral overview of various uses of hydrogenation catalysts.

FIRST COLUMN

The first column is operated with the catalyst located above thehydrocarbon feed. It is operated under conditions to reduce only dienesby hydrogenation. The column is operated to move a key component, forexample C₅, upward into the primary and any secondary beds underconditions of pressure and temperature to hydrogenate only the dienes,e.g., 150° F. to a top temperature of 170-200° F. at 10 to 75 psig. Theexact upper temperature will depend on the diene makeup and otherunsaturates such as the acetylenes and the stability of the particularmix of unsaturates to oligomerization.

The upflow of the key component operates the paradigm shift that keepsthe catalyst clean and inhibits coking of the dienes. The fasterreaction rates of the dienes compared to the mono olefins inhydrogenation allow short superficial vapor phase contact times in therange of 20-60 seconds.

The heaviest carbon-range number that is directed upward serves as thelight-key carbon range for the column. Compounds of lower carbon numberthan this behave as the “lighter than light keys” of the system. Theselighter compounds tend to equilibrate more into vapor than into liquidwhich makes reaction for the dienes in that carbon group much moredifficult. However, use of a secondary reaction zone with catalyst(above at a lower temperature that the primary bed) allows theconcentration of this lower carbon number fraction. Thus, the combinedprimary bed and the optional upper secondary bed together handle a widerboiling range than would otherwise be achievable.

SECOND COLUMN

The second column revives the diene depleted overhead from the firstcolumn and feeds it above the primary and any optional secondary bedswhere it is hydrogenated in the reactive distillation mode. In the nearabsence of dienes e.g. <0.1 wt %, higher temperatures in the range of200-325° F. are used at pressures in the range of 60-150 psig. Monoolefins-only systems tend to have less favorable hydrogen uptake thandiene-rich streams. The superficial vapor phase contact time even underthe more severe conditions is 50-90 second range. Note that the keycomponent carbon numbers in the second column may be coincident with orseparate from the key component carbon numbers in the first columndepending on the objectives of the operation.

The key component builds up in the column liquid and favors the reactionon the key component. In contrast the heavier carbon-range fractionthins out in the downflowing liquid. However, inclusion of a bottomssecondary catalyst bed (which is lower in the column where the highertemperature causes more boil up of the heavies) can be used toconcentrate the heaviest carbon-number range species and react theheavier olefins out more effectively also. As in any distillation thereis a temperature gradient within the distillation column reactor. Thetemperature at the lower end of the column contains higher boilingmaterial and thus is at a higher temperature than the upper end of thecolumn.

The result of the operation of the process in the distillation columnreactor is that lower hydrogen partial pressures (and thus lower totalpressures) may be used.

It is believed that the present distillation column reaction is abenefit first, because the reaction is occurring concurrently withdistillation and the initial reaction products and other streamcomponents are removed from the reaction zone(s) as quickly as possiblereducing the likelihood of side reactions. Second, because all thecomponents are boiling, the temperature of reaction is controlled by theboiling point of the mixture at the system pressure. The heat ofreaction simply creates more boil up, but no increase in temperature ata given pressure. As a result, a great deal of control over the rate ofreaction and distribution of products can be achieved by regulating thesystem pressure. A further benefit that this reaction may gain fromdistillation column reactions is the washing effect, particularly in thedownflow operation of the second column that the internal refluxprovides to the catalyst thereby reducing polymer build up and coking.

Referring now to the FIGURE, a process flow diagram for the removal ofunsaturates, primarily mono- and di-olefins from a full range pyrolysisgas. Such items as reboilers, compressors, pumps and the like have beenomitted but their normal utilization is readily apparent to those in theart.

The feed, a pyrolysis gas as described in TABLE 1, enters the firstcolumn 10 via line 12.

TABLE 1 Component Wt % BD/C4Acetylene 0.2 Butylenes 0.1 Butanes 0.0 C5Saturates 1.0 C5 Olefins 4.2 C5 Diolefins 13.0 C6 Saturates 2.7 C6Olefins 2.3 C6 Diolefins 7.0 C7 Saturates 1.5 C7 Olefins 1.1 C7Diolefins 3.4 C8 Saturates 0.5 C8 Olefins 0.4 C8 Diolefins 1.2 Benzene18.7 Toluene 17.4 EthylBenzene 2.1 Xylenes 7.6 Styrene 2.6 Heavier 12.9Total 100.0

In this illustration the tower 10 is operated under conditions to takethe C₆ fraction upward (bottoms −394° F. top −212° F. at 60 psig.) TheC₇ and heavier carbon atom components are removed via line 34 for otherprocessing. The C₆ fraction contains some C₇ and heavier but iscomprised of predominantly C₆ and lighter carbon number components.

C₆ components contain principally alkanes, benzene, 5 to 12% monoolefins and 15 to 35% dienes. Similarly, the lighter components containa wide distribution of species including dienes and mono olefins.Hydrogen is added via line 14 at a rate to provide an excessstoichiometric amount to the dienes present in the C₆ and higherfraction. In bed 16 a hydrogenation catalyst is provided in the form ofdistillation structure. Under the conditions of temperature and pressuredescribed there is both a vapor and liquid phase comprised principallyof the C₆ components and as a result the C₆ dienes are substantiallyeliminated.

The higher components under these conditions are principally vaporous inbed 16. However, the temperature in bed 18, also a hydrogenationcatalyst as a distillation structure, is lower because of temperaturegradient in the column. The secondary bed 18 allows the lower boilingcomponents to undergo the same type of two phase contact as the C₆fraction in bed 16 thereby allowing a concentration of this higherportion with the dienes substantially eliminated. A side draw 44 is usedto remove a portion of the lights diene-depleted concentrate anddiene-depleted C₆ into collector 40. A portion of the collected materialcan be returned via line 42 (dotted line) to the column 10 to maintainthe vapor load on the column. Otherwise the material from side draw 44is fed to the second column 48 via line 46.

An overhead 20 also containing mostly diene-depleted C₅, C₆ and lightermaterial goes through condenser 22 into collector 24. Thenon-condensibles are removed for recycle to the hydrogen feed 14 or fordisposal via line 26. A portion of the condensed material is returned asreflux 36 to column 10 and the remainder fed via 38 to line 46 intocolumn 48.

The feed from column 10 is characterized as having almost all of thediene and greater unsaturates (acetylenes) removed by hydrogenation withlittle formation of oligomers. The olefins are substantially untouchedbecause of the restricted operating temperature.

In column 48 the operating conditions are more severe in order tohydrogenate the mono olefins (bottoms −338° F., top −251° F. at 100psig). The feed enters above primary catalyst bed 50 which is ahydrogenation catalyst prepared as a distillation structure. Again, theconditions are such that the key component, the C₆ constituents, ismoved downward. The lighter components, primarily C₅+, exit via overheadline 54 through condenser 52 into collector 58. The non-condensibles areremoved either for disposal via line 56 or recycle via line 60 tohydrogen feed 62. A small portion of the liquid in collector 58 isremoved via line 78 and the remainder returned via line 76 to column 48as reflux.

A collector 66 is located on side draw 64 which removes hydrogenatedproduct via line 74. A portion may be returned via line 68 to controlthe vapor load on the column. Alternatively the side draw stream 64 maybe recovered as a vapor (elimination of the collector 66), which,although it will result in an energy penalty, will provide otherbenefits, namely (a) further retention of the heavy olefins in thesecondary bed 72 and (b) a greater increase in the temperature of bed 72relative to the primary bed 50, both of which enhance the performance ofthe secondary bed.

Secondary bed 72 contains a hydrogenation catalyst as a distillationstructure and any heavier fraction remaining is concentrated and given apolishing hydrogenation and recovered via line 70 for combination withthe side draw stream 74 into product stream 80.

Table 2 shows the temperature profile and distribution of the materialsin the column 10. The conditions in the secondary catalyst bed 18(corresponds to trays 4-16) and the primary bed 16 (corresponds to trays17-30) are represented by the blocked out areas. The other trays aredenoted by number. Tray 48 is the reboiler.

TABLE 2 PRESS- LI- PRO- TEMP URE QUID VAPOR FEED DUCT TRAY ° F. PSIALBM/H LBM/H LBM/H LBM/H 1C 101.4 74.7 3510 290.6 vap. 140.2 liq. 2 211.674.7 4912 3940 3 215.8 4902 5343 BED 217.7 75.1 4851 5332 18 219.5 75.34794 5282 221.3 75.5 4730 5225 223.3 75.7 4658 5161 225.6 75.9 4577 5089228.4 76.1 4485 5008 231.9 76.3 4385 4916 236.0 76.5 4278 4815 240.676.7 4170 4709 245.7 76.9 4064 4601 250.8 77.1 3962 4495 255.9 77.3 38634392 260.8 77.5 3266 4294 503.8 Liq BED 265.6 77.7 3184 4201 16 269.477.9 3113 4118 272.4 78.1 3050 4047 274.6 78.3 2995 3985 276.2 78.5 29453930 277.3 78.7 2898 3879 278.1 78.9 2852 3832 278.7 79.1 2808 3787279.2 79.3 2764 3743 279.7 79.5 2719 3698 280.2 79.7 2672 3653 281.079.9 2621 3606 282.2 80.1 2563 3555 284.3 80.3 2489 3497 31 288.1 80.52376 3423 32 296.2 80.7 3532 3311 1206.2 liq.  237.0 vap. 33 310.2 80.93645 3023 34 315.3 81.1 3648 3136 35 319.7 81.3 3645 3139 36 324.1 81.53639 3136 37 328.7 81.7 3632 3131 38 333.6 81.9 3626 3124 39 338.7 82.13621 3117 40 343.8 82.3 3618 3112 41 348.7 82.5 3616 3110 42 353.2 82.73613 3108 43 357.5 82.9 3608 3105 44 361.6 83.1 3597 3099 45 366.1 83.33575 3088 46 371.6 83.5 3532 3066 47 379.8 83.7 3450 3023 48R 393.5 83.92941 508.7 liq.

Table 3 shows the temperature profile and distribution of materials incolumn 48. The conditions in the secondary catalyst bed 72 (correspondsto trays 38-46) and the primary bed 50 (corresponds to trays 19-31) arerepresented by the blocked out areas. The other trays are denoted bynumber. Tray 49 is the reboiler.

TABLE 3 TEMP PRESSURE LIQUID VAPOR FEED PRODUCT TRAY ° F. PSIA LBM/HLBM/H LBM/H LBM/H 1C 131.2 114.7 1504 134.1 vap. 208.7 Liq. 2 251.1114.7 2247 1846 3 255.4 114.9 2268 2590 4 257.3 115.1 2265 2611 5 259.0115.3 2258 2607 6 260.6 115.5 2250 2601 7 262.4 115.7 2240 2593 8 264.3115.9 2228 2583 9 266.5 116.1 2214 2570 10 268.7 116.3 2198 2556 11271.2 116.5 2182 2541 12 273.8 116.7 2165 2525 13 276.4 116.9 2149 250814 279.1 117.1 2132 2492 15 281.8 117.3 2116 2475 16 284.4 117.5 20992458 17 287.0 117.7 2974 2442 643.9 liq. 18 289.9 117.9 2936 2673 BED292.0 118.1 2891 2635 50 293.9 118.3 2846 2590 295.6 118.5 2802 2545297.2 118.7 2758 2501 298.6 118.9 2715 2457 299.8 119.1 2671 2413 301.0119.3 2628 2370 302.1 119.5 2585 2327 303.1 119.7 2541 2284 304.0 119.92497 2240 305.0 120.1 2452 2196 306.0 120.3 2406 2151 307.2 120.5 23582105 32 308.6 120.7 1981 2056 326.2 liq. BED 310.2 120.9 1968 2006 72312.2 121.1 1955 1993 314.4 121.3 1941 1980 316.7 121.5 1928 1966 319.1121.7 1916 1953 321.4 121.9 1906 1941 323.5 122.1 1899 1931 325.4 122.31894 1924 326.9 122.5 1890 1919 328.1 122.7 1888 1915 329.0 122.9 18871913 329.7 123.1 1886 1912 330.2 123.3 1886 1911 330.7 123.5 1886 191147 331.0 123.7 1876 1911 108.5 vap. 48 338.0 123.9 1925 1792 49R 338.3124.1 1841 83.4 liq.

The invention claimed is:
 1. A process for the hydrogenation of olefins,diolefins and acetylenes in a hydrocarbon stream comprising the stepsof: feeding hydrogen and a stream comprising a mixture of hydrocarbonshaving a range of carbon numbers containing olefins, diolefins andacetylene to a first distillation column reactor below a first primarybed and first secondary bed of hydrogenation catalyst wherein a firsthydrocarbon having a carbon number less than the highest carbon numberin said range is boiled upward into said first primary bed along withthe hydrocarbons having a lower carbon number than said firsthydrocarbon, said first hydrocarbon being concentrated in said firstprimary bed and designated the first key component; hydrogenatingsubstantially all of the diolefins and acetylenes contained within saidfirst key component to monoolefins in said first primary bed and aportion of the diolefins and acetylenes contained within thehydrocarbons having a lower carbon number than said first key componentto produce a diolefin depleted stream containing olefins; hydrogenatingsubstantially all of the remainder of the olefins and acetylenescontained within said hydrocarbons having a lower carbon number thansaid first hydrocarbon to monoolefins in said first secondary bed;separating hydrocarbons having carbon numbers higher than said first keycomponent from the diolefin and acetylene depleted stream containing thefirst key component and hydrocarbons having a lower carbon number thansaid key component; feeding the diolefin and acetylene depleted streamcontaining olefins to a second distillation column reactor above asecond primary bed and a second secondary bed of hydrogenation catalystwherein a second hydrocarbon having a carbon number higher than saidfirst key component is concentrated within said second primary bed whilesaid first key component is concentrated below said second primary bed,said secondary hydrocarbon being designated the second key component;feeding hydrogen to said second distillation column reactor below thesecond secondary bed of hydrogenation catalyst; and hydrogenatingsubstantially all of the olefins contained within said second keycomponent and a portion of the olefins contained within the first keycomponents to alkanes within said second primary bed; and hydrogenatingsubstantially all of the remaining olefins contained within said firstkey component to alkanes within said second secondary bed.
 2. A processfor the hydrogenation of olefins, diolefins and acetylenes in a C₅ andheavier hydrocarbon stream comprising: feeding hydrogen and a streamcomprising a C₅ and heavier mixture of hydrocarbons containing olefins,diolefins and acetylene to a first distillation column reactor below afirst primary bed and first secondary bed of hydrogenation catalystwherein the C₆ and lighter hydrocarbons are boiled up into said firstprimary bed and said C₆ hydrocarbons are concentrated in said firstprimary bed and said C₅ hydrocarbons are concentrated in said firstsecondary bed; hydrogenating substantially all of the C₆ diolefins andacetylenes in said first primary bed and a portion of the C₅ diolefinsand acetylenes to produce monoolefins; hydrogenating substantially allof the remainder of the C₅ acetylenes and diolefins to monoolefins insaid first secondary bed; separating the C₇ and heavier hydrocarbon fromthe C₆ and lighter hydrocarbons; feeding the C₆ and lighter hydrocarbonscontaining olefins to a second distillation column reactor above asecond primary bed and a second secondary bed of hydrogenation catalystwherein the C₅ hydrocarbons are concentrated within said second primarybed while said C₆ hydrocarbons are concentrated in said second secondarybed; feeding hydrogen to said second distillation column reactor belowthe second secondary bed of hydrogenation catalyst; and hydrogenatingsubstantially all of the C₅ olefins contained and a portion of the C₆olefins to alkanes within said second primary bed; and hydrogenatingsubstantially all of the remaining C₆ olefins to alkanes within saidsecond secondary bed.
 3. The process according to claim 2 wherein aportion of the acetylene and diolefin depleted C₆ hydrocarbons areremoved as a side draw between said first primary and said firstsecondary beds and the remainder of said acetylene and diolefin depletedC₆ hydrocarbons are removed as overheads along with the acetylene anddiolefins depleted C₅ hydrocarbons.
 4. The process according to claim 3wherein a portion of said overheads are condensed and returned to saidfirst distillation column reactor as reflux.
 5. The process according toclaim 4 wherein a portion of said side draw is condensed and a portionof said condensed side draw is combined with a portion of said condensedoverheads and returned to said first distillation column reactor asreflux.
 6. The process according to claim 3 wherein said side draw iscombined with said overheads as feed to said second distillation columnreactor.